The present invention relates to instruments and methods for implanting a prosthetic joint component. More particularly, the invention relates to instruments and methods for counter boring a long bone for receiving a stemmed and collared implant.
When trauma or disease result in damage to the articular surfaces of a joint, such as the shoulder, it is often necessary to replace the articulating portions of the bones with artificial joint components to restore function and relieve pain. For a long bone such as the humerus, such a replacement involves excising the humeral head to produce a flat seating surface, cannulating the shaft of the bone along the intramedullary canal with a reamer, and seating a humeral implant. Some implants are of a unitary construction with the head being integral with the stem. In that case the implant is seated with the stem in the intramedullary canal and the head seated on the flat surface where the head was resected.
Other implants are of a two-piece design in which the head and stem are connected together at the time of surgery. Two-piece implants provide additional choices with reduced inventory since various head configurations can be interchangeably matched with various stem configurations. Two-piece implants typically include a shoulder, or collar, on the stem that seats on the bone. The head then connects onto the stem above the collar thus isolating the head from the bone. Typically a gap exists between the underside of the head and the resected bone surface. FIGS. 1 and 2 depict a prior art two-piece implant 2 having a stem 4 including a shaft 5 and collar 6. Fins 7 project from the shaft 5 to provide rotational stability to the implant 2. Above the collar 6 is a male taper 8 for engaging a female taper of a head 10. The collar 6 rests against the resected surface 12 of the bone 14 and the shaft 5 lies in the reamed intramedullary canal of the bone. As can be seen in FIG. 2, the collar resting on the bone surface 12 results in a gap 16 between the head 10 and the surface 12.